Backflushing of a capillary column to remove unwanted, less volatile material from the column after the peaks of interest have eluted is a particularly powerful technique and offers significant benefits in reducing analysis time and protecting the column in many applications. The main problem with the existing approach is that there is little control over what portion of the sample enters the column following injection. It is quite possible that heavy sample material may enter a highly retentive column and require extensive temperature programming of the column to enable efficient removal by backflushing. It is possible to minimize the amount of heavy material entering the column by using a programmable temperature vaporizer, enabling sample injection to occur at modest temperatures and allowing the less volatile sample residue to vaporize from the stationary phase during the column backflush step. While this approach has achieved demonstrable success, it does add another level of complexity to the method, the `cut-off` between the target analytes and undesirable less-volatile sample components is not exact, the time taken to cool the injector liner adds to the analytical cycle time and the peak shape of early-eluting sample components may be compromised by matrix-retention effects at the low injection temperature.
A much better way of limiting the ingress of low volatility sample material into a highly retentive separation column is to use a pre-column which may be backflushed in series with the separation column. The use of two, serial columns in this manner mitigates many of the problems associated with backflushing a single column system. The sample is introduced into the pre-column where a crude separation between the light and heavy sample components is performed. The light components elute from the pre-column first and enter the separation column where they are further chromatographed. Once the last component of interest has eluted from the pre-column, the pre-column can be backflushed to remove the heavy material. The advantage of this approach is that the pre-column can be much less retentive than the separation column and heavy material may be backflushed much more effectively.
One of the main problems with the dual-column backflush configuration is that it is difficult to establish the optimum backflush point. Because there is no means of monitoring the peak elution between the columns, it is difficult to establish when all the peaks of interest have passed from the pre-column into the separation column. This time is critical for efficient backflushing. Various methods that have been used over the years include: 1) Running the chromatogram with the precolumn connected directly to the detector. This is not very convenient, and because the pressures will be different from when the backflush is configured, the retention times will not be very accurate. 2) Replacing the separation column with a low dead time restrictor. This is better because it enables the same pressures to be applied as in the backflush method, but is still inconvenient because the process must be repeated each time the GC backflush conditions are changed. 3) Using a monitor detector. This requires additional hardware to split the precolumn effluent. Some of the sample will be lost and an additional detector is required. Despite these disadvantages, it does offer a convenient way of establishing the backflush point.